1. Field of the Invention
The present invention relates in general to an oxygen analyzer of an industrial purpose and more particularly to an industrial oxygen analyzer for detecting oxygen concentration in a combustion exhaust gas in an industrial furnace or boiler or an exhaust gas passageway or in an atmosphere.
2. Related Art Statement
Such a device has been disclosed in the specification of Japanese patent application No. Sho-61-202382 (1986). The detecting principle of the oxygen concentration of said known device will be explained first by referring to a simplified diagram shown in FIG. 9.
An oxygen sensor or sensing element S.sub.o for detecting the oxygen concentration of a measuring gas comprises a diffusion chamber 1 in which the measuring gas is introduced by diffusion, an oxygen pumping portion P.sub.o and an oxygen concentration cell portion B.sub.o being arranged to surround said diffusion chamber, and an air passageway 2 communicating to the outer atmosphere.
The oxygen concentration cell portion B.sub.o compares the measuring gas introduced into the diffusion chamber 1 through a gas introducing hole 3 provided in the oxygen pumping portion P.sub.o with an atmospheric air being a reference air introduced through the air passageway 2 and produces an electromotive force E.sub.o in response to the ratio of the oxygen partial pressure thereof. The produced electromotive force E.sub.o is compared with a reference voltage V.sub.f0 (an electromotive force of about 400 mV corresponding to air ratio m.perspectiveto.1). The deviation or the difference voltage (E.sub.o =V.sub.f0) therebetween is supplied to a PI (proportional integration) controller 4 for controlling the pumping current I.sub.P0 of the oxygen pumping portion P.sub.o so as to compensate said difference or deviation. Since the output of the PI controller 4 is a voltage signal, such a voltage signal is first supplied to a voltage-to-current (V/I) converter 5 and converted into a current amount which is used as the desired pumping current I.sub.p0.
The PI controller 4 acts to control the amount of the pumping current I.sub.P0 through the V/I converter 5 so as to pump out the oxygen in the diffusion chamber 1 by the oxygen pumping portion P according to said deviation (E.sub.o -V.sub.f0). By this control, the oxygen concentration in the diffusion chamber 1 is maintained at 0.002 ppm which corresponds to an air ratio m.perspectiveto.1.
Accordingly, the oxygen concentration of the measuring gas is co-related to the amount of the pumping current I.sub.P0 and by using this relationship, the oxygen concentration of the measuring gas is obtained.
The relation between the gain G.sub.o of the PI controller 4 and the amount of the pumping current I.sub.P0 is given by a diagram shown in FIG. 10. As can be seen from the diagram, there is an oscillating region X.sub.o shown by hatching and an overshooting region Y.sub.o shown by cross-hatching. In both these regions X.sub.o and Y.sub.o in the previous system, stable and accurate measurement of the oxygen concentration of the measuring gas could not be carried out.